HA DYNAMIC OBSTRUCTION OF SORBENTS FOR THE REMOVAL OF MERCURY FROM CHIMNEY GASES FIELD AND BACKGROUND OF THE INVENTION The Emission Standards, as set forth in articles in Amendments to the Clean Air Act of 1990 as established by the Environmental Protection Agency of the United States (EPA), required the estimation of hazardous air pollutants from power plants In December 2000 the EPA announced its intention to regulate the mercury emissions from service kettles heated with coal. Service kettles heated with coal are a major known source of anthropogenic mercury emissions in the United States. Elemental mercury and many of its compounds are volatile and will therefore leave the kettle as constituents in very small quantities in the flue gases of the kettle. Some of these mercury constituents are insoluble in water, which makes them difficult to capture in conventional wet and dry scrubbers. So new methods and processes are needed to capture these constituents in very small amounts of the boiler's flue gases. Mercury appears in the combustion gases from the combustion of mineral coal in both solid and gas phases (mercury confined to particles and mercury in the vapor phase, respectively). The so-called mercury in particle phase is actually mercury vapor phase adsorbed on the surface of ash or carbon particles. Due to the high volatility of mercury and many of its compounds, the majority of mercury found in flue gases is mercury in the vapor phase. Mercury in the vapor phase can manifest as elemental mercury (metallic mercury vapor, elemental) or as oxidized mercury (vapor phase species of various mercury compounds). Speciation, which refers to the form of mercury present, is a key parameter in the development and design of mercury control strategies. All efforts to devise new control strategies for mercury emissions from power plants should focus on this characteristic of mercury. Particle collectors in use in electric service plants, most commonly electrostatic precipitators (ESP) or cloth filters (FF), sometimes called household bags, provide high-efficiency removal of mercury confined to particles. Fabric filters tend to exhibit better particulate-confined mercury removal than ESPs by providing a filter cake on which particulate mercury is trapped as the flue gas passes through the filter cake. If the filter cake also contains constituents that will react with the mercury such as unreacted carbon or even activated carbon, then the filter cake can act as a site to facilitate the gas-solid reactions between the gaseous mercury and the carbon particles. solid. If a power plant is equipped with a Chimney Gas Desulfurization System (FGD) then any of the wet scrubbers or spray dry absorbers (SDA) can remove significant amounts of oxidized mercury. Oxidized mercury, which typically manifests in the form of mercury chloride, is soluble in water, making it available for removal in sulfur dioxide scrubbers. Elemental mercury, insoluble in water, is less likely to be purified in conventional scrubbers. The removal of elemental mercury, therefore, remains a major problem in the search for cost-effective mercury control techniques. Numerous studies have been, and continue to be, conducted to develop cost-effective procedures for the control of elemental mercury. Many of the studies have focused on the injection of a carbonaceous sorbent (for example, pulverized activated carbon, or PAC) in the upstream of the stack gas from the particle collector to adsorb the mercury in the vapor phase. The sorbent, and its charge of adsorbed mercury, are subsequently removed from the flue gases in a downstream particulate trap. Adsorption is a technique that has often been successfully applied for the separation and removal of tiny amounts of undesirable components. The PAC injection is used, commercially, to remove mercury from the exhaust gases of the municipal waste combustion chamber. The PAC injection removes both oxidized and elemental mercury species, although the efficiencies of the removal are higher for the oxidized form. Although this procedure appears attractive at early work, the economy of high injection rates can be prohibitive when applied to service plants heated with coal. More refined studies are now in progress to define more precisely what can and can not be achieved with the PAC. Still other studies seek to increase PAC technology. One technique subjects the PAC to an impregnation process in which elements such as iodine or sulfur are incorporated into the carbonaceous sorbent. Such processes can produce sorbents that bind more strongly with adsorbed mercury species, but also result in the cost of the significantly higher sorbent. The speciation of mercury in the vapor phase depends on the type of mineral coal. Bituminous mineral carbons in the eastern United States tend to produce a higher percentage of oxidized mercury than sub-bituminous and lignite coal from the west. Mineral coals from the west have low chloride content compared to typical eastern bituminous mineral coals. It has been recognized for several years that a flexible empirical relationship is maintained between the content of mineral carbon chloride and the degree to which mercury manifests itself in the oxidized form. Fig. 1 (Source: Senior, C.L. Behavior of Mercury in Air Pollution Control Devices on Coal-Fired Utility Boilers, 2001) illustrates the relationship between the chlorine content in mineral coal and the speciation of mercury in the vapor phase. An important reason for the significant dispersion in the data of Fig. 1 is that the
• Mercury oxidation depends in part on the specific characteristics of the kettle as well as the fuel. The oxidation reactions of mercury proceed through the reaction mechanisms, both homogeneous and heterogeneous. Factors such as the conversion step of the boiler and the temperature profiles of the combustion air preheater, composition of the flue gas, characteristics and composition of the fly ash, and the presence of unburned coal have all been shown to affect the conversion of elemental mercury to oxidized mercury species. Although elemental mercury can be adsorbed to the surface of activated mineral carbon, the capacity is very limited and reversible. That is, the mercury that binds to the mineral coal is a simple adsorption scheme and will eventually develop on the surface of the mineral coal to be re-emitted to the gas phase. If the mercury is going to be permanently captured by the coal, it must be converted (oxidized) to the surface. It has been observed that the reactivity of conventional PAC with elemental mercury vapor is dependent on the presence of certain species of acid gas (eg hydrogen chloride and sulfur trioxide) in the flue gas stream. The presence of hydrogen chloride (HCl), in particular, has been shown to significantly improve the adsorption of elemental mercury from flue gases from the combustion of mineral coal. Hydrogen chloride apparently adsorbs to the surface of the carbon, facilitating the subsequent oxidation of elemental mercury on the surface of the coal. This phenomenon is of great practical importance for the application of the PAC injection for the control of mercury for plants that ignite with sub-bituminous and lignite mineral coals. These mineral coals tend to have a very low chlorine content, and therefore produce combustion gases' which contain only small amounts of hydrogen chloride, and therefore would significantly benefit from the addition of hydrogen chloride in relevant ways. The shortage of halogen-containing gases can be further exacerbated if the PAC injection process is operating downstream of a sulfur dioxide scrubber, such as a wet flue gas desulphurisation system or SDA ("dry"). The scrubber removes acid gases such as hydrogen chloride in addition to the removal of sulfur dioxide. As an example, consider applying the PAC injection to a unit equipped with SDA and a cloth filter that ignites a low-chlorine mineral coal. The concentration of hydrogen chloride in flue gases resulting from the combustion of these mineral carbons is low. This concentration is further reduced by adsorption in the SDA system. This makes the PAC largely ineffective for the capture of elemental mercury in the SDA and the fabric filter. The PAC therefore must be injected sufficiently far from the
, upstream of the SDA to allow the capture of the mercury before the removal of the acid gases in the SDA. This significantly limits the effective residence time available for mercury removal, and requires the use of high carbon injection rates. Felsvang et al. (U.S. Patent No. 5,435,980) teaches that the removal of mercury from a system heated with coal using an SDA system can be improved by increasing the chlorine-containing species (e.g., hydrogen chloride) in the flue gases. . Felsvang and colleagues further teach that this can be achieved through the addition of a chlorine-containing agent to the combustion zone of the kettle, or through the injection of hydrochloric acid (HCl) vapor into the upstream of the gases of the SDA. These techniques are claimed to improve the mercury removal performance of the PAC when used in conjunction with an SDA system. BRIEF DESCRIPTION OF THE INVENTION One aspect of the present invention relates to a still effective method, not expensive to increase the concentration of hydrogen chloride or other halogen-containing compounds, on the surface. of the carbonaceous sorbent as the sorbent is transported to the injection location. Another aspect of the present invention relates to the use of bromine-containing compounds (which the present inventors have determined through the experimental test to be significantly more effective than the chlorine-containing compounds) to improve the capture of elemental mercury by the carbonaceous sorbents. Yet another aspect of the present invention relates to a method of mercury removal that is applicable to virtually all coal-fired power plants, including those equipped with wet or dry FGD systems, as well as those plants equipped only with collectors. of particles. The various features of novelty characterizing the invention are indicated with particularity in the claims appended to and forming a part of this description. For a better understanding of the present invention, its operating advantages and the specific benefits achieved by its uses, reference is made to the accompanying drawings and the descriptive matter in which the preferred embodiments of the invention are illustrated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph that illustrates the relationship between the mercury content of the mineral coal and the mercury speciation for the mineral coals of the United States.; Fig. 2 is a schematic illustration of a first embodiment of the present invention; that is, the Dynamic Halogenation ™ process to treat sorbents for the removal of mercury from flue gases; Fig. 3 is a graph illustrating the removal of mercury achieved through the use of the Dynamic process
Halogenation for treating sorbents according to the present invention through a system comprised of the spray dryer absorber (SDA) and the fabric filter (FF); Fig. 4 is a schematic illustration of a coal-fired electric service plant configuration comprising a boiler and a downstream particle collector; Fig. 5 is a schematic illustration of an electrical service plant configuration; heated with mineral coal comprising a boiler and a downstream spray dryer absorber (SDA) and a particle collector; and Fig. 6 is a schematic illustration of a coal-fired electric service plant configuration comprising a. kettle and a downstream stream collector and a humid flue gas desulfurization system (FGD). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring generally to the drawings, in which similar numbers designate the same or functionally similar elements throughout the various drawings, and to Fig. 2 in particular, a preferred embodiment of the present invention, the Dynamic process, is illustrated. Halogenation to treat sorbents for the removal of mercury from flue gases. As shown in Fig. 2, a system and method according to the present invention comprises a conventional pulverized activated carbon (PAC) injection system 10 includes a storage tank of the sorbent 12 containing a sorbent supply 14, a means for measuring 16 the sorbent 14 in a transport air stream of the sorbent 18, a transport air blower of the sorbent 20 to deliver the air 18 used to transport the sorbent 14 to the injection locations in the (s) chimney (s) s) of flue gas, and a collection point 22 where the sorbent 14 is dispersed in the transport air stream 18. It should be recognized that this is only one mode of the pneumatic transport conveyor system, and many other configurations could be use or develop with one of ordinary skill in the art without departing from the scope of the present invention. The key aspect of the present invention is that the halogen-containing reagent or compound 24, which may be in gaseous form, is injected into the air stream / flowing transport sorbent at a point 26 near point 22 where 14 and the air Transport 18 is first mixed together. The adsorption of the halogen-containing reagent 24 on the particles of the sorbent 14 occurs during the transport of this gas mixture from a solid to the injection point 28 in a dynamic process. The proportion of halogen adsorption during this transport is relatively high due to the locally high concentration of the halogen in the transport line. Once the sorbent enters the chimney or SDA the desorption and halogen ratio of the carbon surface is very low compared to the residence time for the reaction with mercury to lose significant amounts of halogen back to the gas phase. This is because the inventors refer to the present invention and to the process as Dynamic Halogenation. This design maximizes the available residence time for the halogen-containing compound 24 and is adsorbed on the surface of the sorbent 14 before the sorbent 14 is injected into the flue gas stack (s), the injection locations they are generally designated 28. This process maximizes the benefit and utilization of the halogen-containing reagent 24 by placing it exactly where it is needed to facilitate the removal of the elemental mercury on the surface of the sorbent 14. The particles of the sorbent 14 with its reagent charge containing adsorbed halogen 24 enters the injection locations of the flue gas stack (s) 28 with high reactivity for the removal of the elemental mercury. The present invention is advantageous for the prior art methods. The removal of an elemental mercury from the coal combustion gases generated by electric utility plants through the application of a conventional PAC injection process is very expensive. The present invention promises to significantly reduce the cost of mercury removal in electric plants fired with coal. First, the process provides the benefits, in terms of reactivity with elemental mercury, of replacing a pretreated, expensive PAC solvent (eg, PAC impregnated with iodine) with a low cost, conventional sorbent. The present invention is an improvement over Felsvang et al. (U.S. Patent No. 5,435,980) because the present invention makes the use of the halogen-containing reagent 24 much more efficient by placing it on the surface of the carbon sorbent 14 just prior to injection of the flue gases. In the transport line, the sorbent does not have to compete with the alkaline fly ash or SDA lime suspension for the available halogen gas. It has been found by the inventors, and by several other investigators, that the addition of hydrogen chloride gas to stack gases separately from the PAC injection system, as was thought by Felsvang et al., Does not significantly improve the removal performance of elemental mercury and the PAC injection process. This is due to the fact that much of the injected hydrogen chloride reacts with other constituents of flue gas (eg calcium compounds contained in the coal ash particles in flight), thus preventing halogen from adsorption on the sorbent and thus increased the performance of the injected PAC. By making the use of the halogen-containing reagent 24 efficient, the present invention allows much lower addition ratios for the halogen-containing reagent 24 relative to other methods for the addition of halogen. The present invention also has a significant advantage over other means of adding halogen-containing compounds 24 to flue gases in which the boiler and other power plant components are not subjected to the corrosive nature of the halogen compounds. This is substantially true when compared to the addition of halogen to the combustion chamber of the kettle. High-temperature corrosion of the kettle components by chlorides is a well-known and serious problem. • The present invention was tested in a Small Kettle Simulator Facility (SBS) of 5 million Btu / hr. The SBS went on to approximately 4.3 million Btu / hr with a sub-bituminous mineral coal from the eastern United States. During these tests the chimney gases that exist in the first SBS boiler are passed through a dehydrating dryer absorber (SDA) for the removal of sulfur dioxide, and then through a cloth filter (FF) to remove the sulfur dioxide. the removal of ash in flight and the solvent exhausted from the FGD system of SDA. A dynamically Halogenated PAC stream, prepared by the method of the present invention, was injected downstream of the stack gas stream of the SDA, upstream of the fabric filter. Hydrogen bromide (HBr), hydrogen chloride and the chlorine gases were each examined. All were effective, but the HBr was not very effective. The halogen-containing reagent 24, and a commercially produced PAC were used as the carbonaceous sorbent 14. FIG. 3 illustrates the removal of the mercury through the SDA / FF system during the operation of the Dynamic Halogenation process with HBr. It can be seen that the injection of the dynamically halogenated PAC, the mercury in the vapor phase that exists in the system, decreased from its initial value of approximately 6 μg / dscm to well below 1 μg / dscm. Other significant observations included: 1) PAC injection, alone, in a proportion of the similar injection provided not discernible from mercury removal; 2) the use of hydrogen bromide was no more effective than the use of hydrogen chloride; and 3) the addition rates of both hydrogen bromide and PAC were many times lower than the proportions for other halogen addition processes and conventional PAC injection processes, respectively. Conventional PAC injection may require 4,536 kg (10 pounds of PAC or more per cubic foot per million stack gas to achieve 90% mercury control compared to 0.27216 kg (0.6 pounds) per cubic foot per million of stack gas using the subject invention The amount of halogen gas required to affect this improvement is in the order of one thousand times less than would be required by direct injection of the halogen gas directly into the chimney or SDA.These results indicate that the present invention offers a very cost-effective method of removing elemental mercury from coal combustion flue gases Based on the conducted test, it is believed that the desired levels of mercury removal will be achieved by providing (using commonly used terms) in the power generation industry) the halogen-containing reagent 24 in a ratio equivalent of up to about 4 moles of halogen per million moles of stack gas, and by providing at least about 0.1 pounds of sorbent 14 per cubic foot per million of stack gas. In the preferred embodiment illustrated in Fig. 2, the halogen-containing reagent 24 is either hydrogen bromide or bromine (Br2), and the carbonaceous sorbent 14 and the halogen-containing reagent 24 are carried together in the transport line pneumatic of the sorbent with sufficient residence time for the halogen-containing reagent 24 to be adsorbed on the carbonaceous sorbent particles 14 before the sorbent 14 is injected into the coal-combustion flue gas stream. Based on the conducted test, it is estimated that a residence time of approximately 0.5 to approximately 1.0 seconds was achieved. In yet another embodiment the boiler fuel heated with mineral coal may include bituminous, sub-bituminous and lignite coals and mixtures thereof. The present invention is not limited to applications where the mineral coal is being burned. It can also be applied to any type of combustion process where mercury solutions are going to be controlled, such as in relation to combustion processes involving combustion and municipal solid waste in incineration plants. In yet another embodiment, the reagent containing bromine 24 could comprise hydrogen bromide (HBr) or bromine (Br2) gas. In still another embodiment, the halogen-containing gases 24 may include any one or more of the following: hydrogen chloride, chlorine (Cl 2), as well as fluorine and iodine compounds, and halides derived therefrom. In yet another embodiment, the carbonaceous sorbents 14 may include, but are not limited to, powdered activated carbon (PAC), charcoals and charcoal produced from mineral coal and other organic materials, and unburnt carbon produced by the process of combustion itself. In yet another embodiment, electrical utility plant configurations may include plants equipped with only a particle collector (FF or ESP) (Fig. 4); a FGD of SDA and a particle collector (FF or ESP) (Fig. 5); or a particle collector (FF or ESP) and a wet FGD (Fig. 6). In yet another embodiment, the spent carbonaceous sorbent can be removed separately from the coal fly ash, if desired, by adding an additional particulate collector specifically designated for the capture of the injected amount of the carbonaceous sorbent. The present invention takes advantage of the ability to dynamically halogenate the carbonaceous sorbent 14 at the site, in the utility plant heated with mineral coal, as needed, thus avoiding any of the manufacturing processes off-site produced. Conventional pneumatic conveying equipment can be used, and the mixing of the halogen-containing reactant stream 24 and the carbonaceous sorbent stream 14 can take place under typical environmental conditions for such equipment at a power plant site, for example from about 0 ° C to about 50 ° C. While the specific injection locations 28 where the combined stream of the halogen reactant and the carbonaceous sorbent can be injected into the mercury-containing flue gas, several locations will suffice. Such a location could be in the flue gas stream precisely downstream (with respect to the direction of the flue gas flow through the installation) of the air heaters conventionally used in such power plants, ie, in the location 28A as illustrated in Figs. 4, 5 and 6, where the stack gas temperature is typically approximately 150 ° C, but the stack gas temperature in such a location 28A could be up to about 175 ° C or as low as about 120 ° C. . Another such location could be in the flue gas stream at location 28B as illustrated in Fig. 5, which is just upstream of the particle collector devices (FF or ESP), but downstream of the SDA apparatus. While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes can be made in the form of the invention covered by the following claims without depart from such principles. For example, the present invention can be applied to the new construction of the fossil fuel boiler that requires the removal of mercury from the flue gases produced in this way, or to the replacement, repair or modification of existing fossil fuel boiler facilities . The present invention can also be applied, as described above, to new incinerators for the combustion of MSW or to the replacement, repair or modification of existing incinerators. In some embodiments of the invention, certain features of the invention can sometimes be used to favor without a corresponding use of the other features. Accordingly, there are other alternative embodiments that would be apparent to those skilled in the art and based on the teachings of the present invention, and which are proposed to be included within the scope and equivalents of the following claims of this invention.